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Unformatted text preview: MECH 340 Engineering Material Mechanical Properties Area, A F t F t Objectives • Introduce the basic concepts associated with mechanical properties of materials. • Evaluate factors that affect the mechanical properties of materials. • Review some of the basic testing procedures that engineers use to evaluate many of these properties. Terminology • Stress Force or load per unit area of crosssection over which the force or load is acting. • Strain Elongation change in dimension per unit length. • Young’s modulus The slope of the linear part of the stressstrain curve in the elastic region, same as modulus of elasticity. • Shear modulus (G) The slope of the linear part of the shear stressshear strain curve. • Viscosity ( ) Measure of resistance to ¡ow, de¢ned as the ratio of shear stress to shear strain rate (units Poise or Pas). • Thixotropic behavior Materials that show shear thinning and also an apparent viscosity that at a constant rate of shear decreases with time. Common States of Stress • Simple tension: cable o σ = F A A o = cross sectional Area (when unloaded) F F σ σ • Simple shear: drive shaft o τ = F s A τ Note: τ = M / A c R here. M M A o 2R F s A c Common States of Stress Canyon Bridge, Los Alamos, NM • Simple compression: A o Balanced Rock, Arches National Park o σ = F A Note: compressive structure member ( σ < 0 here). Common States of Stress • Biaxial tension: Pressurized tank σ z > 0 σ θ > 0 • Hydrostatic compression: Fish under water σ < 0 h Response to Applied Load The response of engineering materials to an applied stress can take many form depending on the structure and properties of the stressed material pure elastic elastic plastic viscoelastic rubbery viscous creep relaxation Newtonian and non Newtonian Liquids Shear stress strain rate relationships for Newtonian and nonNewtonian materials The Tensile Test Load  The force applied to a material during testing. Strain gage or Extensometer A device used for measuring change in length and hence strain. Glass temperature (Tg ) A temperature below which an otherwise ductile material behaves as if it is brittle. Engineering stress  The applied load, or force, divided by the original cross sectional area of the material. Engineering strain  The amount that a material deforms per unit length in a tensile test. Tensile Test The stressstrain curve for an aluminum alloy Example An aluminum rod is to withstand an applied force of 45,000 pounds. To assure a sufFcient safety, the maximum allowable stress on the rod is limited to 25,000 psi. sufFcient safety, the maximum allowable stress on the rod is limited to 25,000 psi....
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This note was uploaded on 02/07/2011 for the course MECH 340 taught by Professor Marwandarwiche during the Summer '07 term at American University of Beirut.
 Summer '07
 MarwanDarwiche

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